1. Field of the Invention
This invention relates to the detection of trace levels of organic pollutants using Raman spectroscopy. More particularly, this invention relates to using polymer substrates to preconcentrate analytes to improve the sensitivity of Raman spectroscopy.
2. Description of the Related Art
Many techniques are known for detecting organic compounds in situ in vapor and/or liquid phase. However, all of the available techniques have some shortcomings. One problem common to many analytical techniques is the need to regularly calibrate the sensor using some reference standard. It would be desirable to have a sensor that could be internally calibrated.
Raman spectroscopy is an advantageous technique for detecting organic compounds, due to its high selectivity. A Raman signal can be associated with a unique organic compound. Raman spectroscopy works by laser probe light exciting molecules to higher energy states, often in their vibrational energy bands. As these molecules return to equilibrium, they emit characteristic Raman signal photons. This emitted light can then be analyzed to determine the specie and its amount in the solution.
Unfortunately, Raman spectroscopy has a fairly high lower limit of detection (LLD). One method that has been used as a work-around to this high LLD is surface enhanced Raman spectroscopy, or SERS. SERS improves the LLD by using a substrate with a thin metallic coating that essentially irreversibly adsorbs the analyte, to preconcentrate the analyte. A good review of SERS can be found in Keith T. Carron's dissertation, Surface Enhanced Resonance Raman, Resonance Hyper-Raman, and Hyper-Raman Spectroscopy of Molecules Absorbed to Thin Metal Films, Northwestern University 1985.
SERS has its own drawbacks. This thin metal coating must be prepared with great care to achieve optimal results. SERS is also not generally available: only certain species of organic compounds are amenable to detection using this technique. Also, a SERS sensor substrate generally can be used only once, because the binding between the substrate and the analyte is essentially irreversible.
Resonance Raman spectroscopy is another technique that has been developed to improve the limits of detection. However, Resonance Raman spectroscopy is not suitable for in situ analysis.
Fiber optic infrared spectroscopy provides in situ real time analysis, and is highly selective. However, there are problems with the available optical fibers that transmit infra-red light. These fibers have a limited optical window, particularly with extended fibers. CCl.sub.4, for example, cannot be detected with an extended fiber. Trichloroethylene is marginally detectable. These fibers also lose a large fraction of signal during transmission. Currently available IR optical fibers have power losses ranging from 1,000 to 10,000 dB/km. Even assuming a tenfold improvement in the power loss of IR optical fibers, to between 100 and 1,000 dB/km, this would still be much higher than the 0.2 to 10 dB/km power loss that is typical for silica optical fibers. Further, no couplers are currently available for these fibers, making multiplexing problematic.
Gas chromatography is highly selective, is amenable to use with a broad range of analytes, and has an excellent LLD, especially when coupled with mass spectrometry. However, gas chromatography is not an in situ technique, and errors can be introduced by improper sampling.
Another technique that has developed for chemical sensing is surface acoustic wave analysis, or SAW. SAW analysis works by measuring the change in the vibrational frequency of a thin polymer film, as analyte molecules adsorb or desorb from the polymer film. See U.S. patent application Ser. No. 07/970,750, incorporated in its entirety by reference herein. SAW has its own drawbacks, however. It is not very selective, and it does not work in liquid phase analysis.
A consequence of the extensive research that has been done on SAW analysis is that a large number of polymers have been identified as selective adsorbents for particular organic analytes. See generally, D. S. Ballantine, Jr., S. L. Rose, J. W. Grate, H. Wohltjen, Analytical Chemistry 58 3058-66 (1986), and references therein, incorporated by reference herein. See also R. A. McGill et al., "Choosing Polymer Coatings for Chemical Sensors, CHEMTECH 24 (9) 27-37, and references therein, incorporated by reference herein. Despite this effort, SAW analysis remains relatively poorly selective. Table I lists a few of these polymers, and the organic species that are selectively adsorbed by them:
TABLE I ______________________________________ POLYMER ANALYTE ______________________________________ poly(isobutylene) TCE, chloroform poly(cyanoallyl)-siloxane methylene chloride poly(phenylether) benzene fluoropolyol dimethyl(methylphosphinate) ______________________________________